This invention relates to a process for preparing particles of bioabsorbable polymer, e.g., spheroidal particulates or beads, employing the technique of rotary atomization. The particles are useful, inter alia, in medical diagnostic procedures such as mammography and in the repair of damaged or defective bone.
The medical use of polymer particles including those of the bioabsorbable variety are known, inter alia, from U.S. Pat. Nos. 3,882,858; 4,535,485; 4,547,390; 4,643,735; and 4,663,447. Particles of bioabsorbable polymer have been utilized to facilitate bone or fibrous tissue repair/reconstruction.
The problem of repairing defective bone has presented difficulties for some time. Until recently, the only practical technique was to immobilize damaged or broken bones and to rely upon nature to effect regrowth of skeletal tissue into a cavity in bone. With the advent of surgical techniques, it has become possible to utilize implanted bone substitutes, i.e. prostheses, to replace injured or diseased bone structure and to repair congenital or degenerative defects in the skeletal structure. The processes of bone formation and renewal are carried out by specialized cells. Bioabsorbable polymer particles constitute one type of such prosthesis facilitating the regrowth/rejuvenation of surrounding bone.
More specifically, such prosthesis comprises a quantity of bioabsorbable polymer particles which initially are not bonded together. The particles can be inserted into areas of bone defects such as cavities, or packed into any hard tissue part of the body in which repair, reconstruction, or replacement is desired. Areas where the loose individual bioabsorbable polymer particles can be applied include, e.g., dental applications such as filling in diseased recessions or pockets within a tooth or between adjacent teeth, and also non-oral applications including fracture healing, filling of defective bone areas, bone replacement, etc.
The bioabsorbable polymer particles in loose form can be initially packed into cavities or other areas of hard tissue defect or disease. When a void in a hard tissue area is completely filled with the loose, individual particles of bioabsorbable polymer, the surrounding soft tissue can be sutured over the particles to completely enclose them. Once enclosed, the particles come into contact with each other, forming pores that are readily wettable by blood and body fluids. In this way, bone growth is promoted and bone recession prevented. As the healing process progresses, ingrowth of surrounding bone and/or tissue throughout the interstices of the packed particles occurs, with the bioabsorbable particles gradually being resorbed by the growing bone or tissue and resulting in healed bone or tissue structure. Thus, the bioabsorbable particles serve as a matrix support for active replacement of missing bone or tissue. More particularly, these implanted particles of bioabsorbable polymer serve as both a structural support and as a guiding matrix for encroaching bone deposits derived ultimately from adjacent fresh bone.
With the increase in interest in utilizing bioabsorbable polymer particles to facilitate bone or fibrous tissue repair/reconstruction, the need to provide such particles with optimum shape, dimensions and restorative or diagnostic properties has also increased. For example, such particles of bioabsorbable polymer should be prepared with as uniform sizes as possible, in order to optimally effect the restorative procedures.
Because of the hygienic nature of application, the bioabsorbable polymer particles must be maintained in decontaminated form. In other words, the polymer and resulting particles cannot become contaminated, so that processing of the bioabsorbable polymer, including breakup into the individual particles thereof, must be carried out under totally sanitary conditions. Therefore, processing of the bioabsorbable polymer into particles must be carried out using the polymer alone or with relatively little solvent present, i.e., when the polymer is in viscous state.
A number of processes are known for preparing finely divided polymeric particles, e.g., mechanical grinding, solvent precipitation, dispersion, spray atomization of solutions or slurries and rotary atomization. In rotary atomization, the polymer is applied to a rotating bell, cup, or disk, with mechanical forces predominating in the breakup of the polymer into particles. More specifically, the polymer is introduced near the center of the rotating bell, cup, or disk whereby centrifugal force distributes the polymer to the edge of the rotating bell, cup, or disk, at which the polymer has an angular velocity close to the angular velocity of the rotating bell, cup, or disk. As the polymer leaves the surface of the rotating bell, cup, or disk at the outer edge thereof, a principal velocity component thereof is tangential to the rotating member, and the polymer is spun off in the form of a thin sheet or small cusps. The flowable polymer is then atomized by turbulent or aerodynamic disintegration, depending upon conditions. Generally, viscosity of the polymer being atomized is as low as possible to enhance atomization.
Examples of rotary atomizers can be found in U.S. Pat. Nos. 4,256,677; 3,743,464, and 3,741,703. For instance, a rotating disk having an outer peripheral edge of teeth is disclosed in U.S. Pat. No. 4,256,677. By rotating this toothed disk at a constant speed, centrifugal force necessary to pinch off particles clinging to the peripheral teeth of this disk remains constant, hence all particles pinched off the peripheral teeth have substantially the same size. Particles formed by using a toothless disk would not be as uniform in size. U.S. Pat. No. 3,743,464 discloses an apparatus for sphering small particles which comprises a plurality of concentric, radially-spaced cylinders and a rotating plate underneath, with material introduced into the innermost cylinder being gradually sphered and propelled through openings into adjacent outer cylinders. U.S. Pat. No. 3,741,703 relates to improving uniformity of particle size generated during rotary atomization by turning upwardly the peripheral edge of the rotating atomization plate, whereby the particles of material broken up on the atomization plate are subjected to a more uniform flow and even treatment.